Dysfunctional retinoid signalling leading to aberrant differentiation and uncontrolled proliferation, has been found to be linked with several forms of cancers. This proposal focuses on understanding the mechanisms of retinoid-induced differentiation and growth arrest in P19 teratocarcinoma cells. Being a tumoral line, P19 cells can be used as an in vitro model system to study the anti-proliferative and differentiation-inducing effects of retinoids. Additionally, their ability to undergo terminal differentiation into several distinct cell types upon RA treatment, makes them invaluable to study the mechanisms responsible for these effects. The effects of RA are mediated by two distinct classes of nuclear receptors - the retinoic acid receptors (RARs) and the retinoid X receptors (RXRs) which belong to the superfamily of ligand-inducible transcription factors. Due to a high degree of functional redundancy between RARs, gene-disruption studies in mice have not provided significant information on their role in regulating cell differentiation or proliferation. Presently, gene-disruption studies of RARs in the RA- responsive embryonal carcinoma (EC) cells (F9 and P19), and the use of receptor-type selective retinoids, offer the most successful means to directly determine the role of RARs in differentiation and proliferation. Using these two approaches in F9 cells, we have identified the individual roles of RARalpha and RARgamma in the formation of two distinct endodermal cell types. More importantly, we have shown that transcriptional activity of RARs for differentiation into distinct cell types, is dependent on phosphorylation by proline- directed kinases and by PKA at specific sites, indicating that RARs provide a focal point of cross talk between different signalling pathways. We will now determine the role of RARs and the modulation of their activity by phosphorylation, in the formation of several distinct lineages in P19 cells, in response to both natural and synthetic retinoids. RA is presently used therapeutically as an anti-tumoral agent due to this anti-proliferative and differentiation inducing capacities. Defining the basis of retinoid receptor function in controlling cell growth and differentiation, with the identification of RA-target genes involved in these processes, is crucial to understanding, and exploiting, the effects of retinoids for cancer therapy. From a health standpoint, the results obtained from the proposal should have an impact on the use of retinoids for cancer therapy, and help extend our understanding of cancers that arise due to defective retinoid signalling.